1. Field of Invention
Coal water slurry composition based on low rank carbonaceous solids.
2. Prior Art
The basic coal water mixture fuel technology has emerged as one potential answer to uncertain fuel oil supply to the electricity generating industry during the past two decades. As is evident from numerous patents in the field, coal water mixture technology provides methods to manufacture fuels based on finely pulverized coal and water and wherein different chemicals are used to enhance both the solids concentration of the fuels and the pumpability and combustibility of such slurry fuels.
Although it may well be said that coal water mixture fuels have proven technically and economically useful alternatives to fuel oil, it is equally true that considerable improvement over the present state of the art is required to fully utilize the potential benefits of the coal water fuel concept.
For example, U.S. Pat. No. 4,282,006 discloses a combination of particles of a claimed unique particle size distribution with dispersing chemicals and water to form particularly advantageous slurry fuels, and U.S. Pat. No. 4,358,293, discloses the use of nonionic surface active materials incorporating a hydrophobic portion and a hydrophilic portion comprising at least 100 repeating ethylene oxide units to form slurries of coal in water. In particular, the patent discloses a method of cleaning the finely divided coal which includes a pretreatment of the coal particles with various chemicals and oils to render their surfaces oleophilic and hydrophobic. Thus, cleaned coal is slurried with water and nonionic surfactants to yield directly burnable fuels. The fuels may also contain various salts and polymeric stabilizers which serve to keep the particles of pretreated coal in suspension.
U.S. Pat. No. 4,470,828 discloses compositions of coal water slurry fuels which have enhanced stability and pumpability. Combinations of certain chemicals are responsible for producing these advantageous effects. Thus, the patent teaches the use of particular anionic surface active agents in combination with either polyether polyols or esterified such compounds, or phosphated, sulphated or carboxylated such compounds.
Such teachings show that functioning slurries of fine coal in water can be made by paying attention to particle size distribution, coal surface conditioning and selection of dispersing chemicals. One patent, U.S. Pat. No. 4,501,205, even teaches the blending of different coals into the slurry. The patent teaches (col. 7, lines 17-25) the use of at least one coarse carbonaceous fraction, such as anthracite or low volatile bituminous coal and at least one fine carbonaceous fraction such as lignite, to make up the slurry solids. In other words, the patent teaches the use of at least two types of coal to make up the solids of the slurry whereby the fine fraction is a lower rank coal and the coarser fraction is a higher rank coal.
A particular weakness of present day coal-water mixture fuel technology appears to be a greatly increased need and decreasing efficiency of dispersing chemicals with decreasing rank of the coal involved. This predicament is doubly unfortunate because the mining cost of lower rank coals, i.e., coals with relatively high oxygen content, is normally significantly lower than that of high rank coals, i.e., coals with relatively low oxygen content. Slurry fuels based on lower rank coals could potentially offer very significant economic benefits as replacement fuels for increasingly scarce fuel oil produced from finite supplies. Lower rank coals as slurry fuel feedstock further offer the distinct environmental advantage of containing very low sulfur concentrations; in fact, lower than the high rank coals which are presently used as suitable coal-water mixture fuel feedstock.
Recent publications, such as "Effects of coal type, surfactant, and coal cleaning on the rheological properties of coal water mixtures" (Kaji et al., 5th International Symposium on Coal Slurry Combustion and Technology, U.S. DOE, Tampa, Fla., Apr. 25-27, 1983) clearly show that coals with higher oxygen contents, commonly exceeding some 6 to 8 weight precent in the ultimate analysis (ASTM procedure D3176-74 (1979), dry and mineral matter free basis) require high levels of addition of surface active dispersing agents before they flow, even at very low solids concentrations. In fact, using nonionic surface active dispersing agents in accordance with U.S. Pat. No. 4,358,293, or anionic dispersing agents as taught in e.g., U.S. Pat. No. 4,504,277, it has been found that lower rank coals, i.e., coals with more than 6-8% oxygen (ASTM D3176-74, dmmf) frequently require high amounts of dispersing agent. Such high addition levels bring the cost of manufacturing the slurry fuels to a level where the commercial feasibility suffers.